Blow lance assembly for metal manufacturing and refining

ABSTRACT

The instant invention relates to a blow lance assembly for metal manufacturing and refining, developed so as to control slag formation and oxidation, as well as the heat capacity of the reactor, and the conservation of the operational conditions during charging and blowing, having, in its lower part, two groups of gas outlets which determine two blowing conditions, the first group consisting of oxygen passage nozzles having a converging-diverging shape, main responsible for the oxidation reactions and for the conveyance of the basic solid material, mainly calcium oxide, for initial slag formation, and dephosphorization at the final stages during batch refining; the second group consisting of secondary jets with various functions during each blowing stage, the first function, at the beginning of the process as an afterburning agent, through the reaction of oxygen with carbon monoxide generated by the main jets, and the second function being that of accelerating the reaction with carbon by increasing oxygen jet speed, accelerating scrap melting in the early stages and, finally, incrementing the oxidation of the elements of the metal bath, iron, in order to reduce the phosphorus content in the final stages of batch refining.

The instant invention relates to a blow lance assembly for metal manufacturing and refining, more specifically to a blow lance assembly used in steel manufacturing and refining, developed so as to control formation and oxidation of slag, heat capacity of the reactor, and conservation of charging and blowing operational conditions.

STATE OF ART

The BOF (Basic Oxygen Furnace) furnace is a cylindrical vessel closed at the bottom, having its upper end shaped like a conical frustum with a large opening at the top for charging the liquid pig iron and scrap, called “mouth”, and a small side opening called “pouring channel” through which liquid steel produced at the end of the primary refining is removed.

To protect the furnace metal housing, a coating with a layer of refractory bricks is used in order to contain the liquid bath at elevated temperatures around 1700 Celsius degrees.

The blowing process involves performing a sequence of steps, starting with charging. The vessel is tilted to an angle of 45° from the vertical; scrap is charged into the vessel with the aid of a channel, a container for preparing the scrap to be placed into the furnace; after the scrap is placed into the furnace, the liquid pig iron is charged. The vessel is then tilted back to the vertical position, to allow oxygen to be blown through an oxygen lance which moves vertically.

The oxygen lance is water cooled, and contains at its end the oxygen outlet nozzles. The nozzle assembly and its geometry determine the configuration of the lance nozzle. The oxygen lance follows a height pattern in relation to the metal bath during the blow, called “lance-bath distance”. The objective is always to approach the lance closest to the surface of the bath to accelerate the reaction speed, being, however, as close as possible to the bath surface subjected to high temperatures. On the other hand, the closer to the bath surface, the deeper the injection of the oxygen jets, which increases the reaction speed.

The process causes agitation of the liquid metal and slag that are thrown to the upper parts of the furnace and may solidify both on the lance and along the furnace walls, and may also be thrown out of the furnace. Besides oxygen, the lance can also use other gases or mixtures thereof with oxygen in liquid metal manufacturing processes.

To obtain a blow lance with longer service shelf life, it must be refrigerated, as in this case, through water circulation. Temperatures in the outer face of the lance are high and much greater than the boiling point of water. If the metal to be processed is steel, temperatures exceed 1700° C. and in all processed batches the lance is immersed in a mixture of liquid bath, slag and gas called “emulsion”.

The blowing process consists of four distinct steps: ignition, slag formation, decarburization and oxidation for setting the temperature. To initiate the process, the lance is lowered to a height that allows the ignition of the batch, i.e, in which oxidation occurs in any element of the bath by blown oxygen. Immediately after batch ignition, the slag formation step begins. This second step lasts approximately 3 to 5 minutes, and it is also referred to as the first period of decarburization. It is characterized by the almost complete oxidation of silicon and a sharp manganese oxidation, while the decarburization speed increases as the content of these two elements decrease. During this initial step of slag formation, all the slag-forming agents, such as calcitic lime, dolomitic lime and raw dolomite, are added.

The addition of slug-forming materials is generally performed using storage silos located above the converter furnace. The supply logistics of these silos is complex, comprising several stages, including the receipt of the material through highways or railways, carried in bulk or in “big-bags” (large packs). When carried in bulk, the material is discharged in transfer silos, usually located in an opening below the transport means; from the wait silo, the material is dosed through a hopper and moves towards a conveyor belt whose function is to lead the material to the top of the bearing constructions of the converter furnace with heights between 25 and 50 m, where storage silos are located; during the ascent, belts transposition may occur to allow changing the direction of the material depending on the layout of each company. At the top, the materials reach a larry called tripper. The material is then directed to storage silos, usually 4 to 15 in number. Beneath the storage silos there are vibrators or dosers that, upon receipt of the weighing command, move the material to the waiting silos, which are provided with a scale for weight adjustment. The heavy stuff is awaiting the right time to be added into the converter furnace. In case big bags are employed, they can be opened in transfer silo or lifted by overhead crane, and unloaded directly on the storage silos. In both cases, in each material transposing, the pollution effect is remarkable and containment requires considerable investment in dust-removal systems.

These materials can also be added by lances or through porous or pressurized passages existing at the refractory sole or base of the furnace. The moment of addition varies depending on the type of steel to be produced, but in general, follows the same sequence. As accelerated oxidation occurs, silicon being transformed into silica, the rapid addition of a basic agent is necessary in this case, and especially lime, added immediately after finding that the batch ignition was carried out. As the material is granulated, time is required for its heating, reaction and dissolution, and then for the occurrence of an effective silica neutralization action. Then, magnesium oxide-rich materials, such as dolomite, are added with the main objective of obtaining a saturation level of slag such that avoids the attack to the refractory bricks of the converter. The behavior of magnesium oxide-rich materials is the same as lime and, depending on the silicon content in pig iron, lime dissolution mastery is critical to avoid overflow of the emulsion to the outside of the converter or its projection, which would have harmful consequences on the batch performance, operating time, formation of solid metallic materials adhered to the lance, and on the dust-removal system, which imposes long pauses for maintenance.

The second stage of decarburization mainly involves the oxidation of carbon, after silicon oxidation. The conditions in the converter are characterized by high temperature and by the existence of gas-slag-metal emulsion which promotes decarburization, the reaction speed being determined only by oxygen availability. The furnace operates as an autothermal reactor in which the energy required for the process is provided by the liquid charge, pig iron, and by the refining reactions resulting from the reaction with oxygen.

The oxidation reaction forms two products: carbon monoxide (CO) and carbon dioxide (CO₂), with levels ranging from 40 to 70% CO and 10 to 40% CO₂. The intense generation of carbon monoxide within the metal bath causes the “foaming” of the slag and the formation of the gas-slag-metal emulsion. The afterburning technique of the gases inside the furnace aims to oxidize carbon monoxide into carbon dioxide and generate a substantial amount of energy. The efficiency of transmission of this additional amount of energy to the charge may also affect the amount of scrap used. The increased proportion of scrap in the charge, and thus, the production of steel per ton of liquid pig iron, requires an adjustment in heat balance, making use of additional energy sources necessary. Scrap preheating and the addition of auxiliary fuel such as iron-silicon and metallurgic coke are traditional.

Decarburization reactions are exothermic and increase the temperature of the metal bath. The end of this step is determined when the decarburization speed is now controlled not by oxygen availability, but rather by the diffusion of carbon to the reaction interface. Afterburning is maximized during batch decarburization, associated with the conveyance of gases into the furnace atmosphere and their entraining into the primary or secondary oxygen jets. The entrained carbon monoxide is oxidized into carbon dioxide. A carbon dioxide portion is dissipated to the furnace atmosphere, and the remainder reaches the bath and the emulsion, being reduced again by the metal. The lances' nozzles specially designed for afterburning are characterized by the existence of two oxygen blowing conditions: the main blow by the convergent-divergent nozzle, and the supplemental oxygen blow through straight nozzles, called secondary jets.

The last blowing step aims to increase the temperature of the metal bath, particularly in processes where heat input is compromised by larger amounts of scrap placed into the furnace. This step is characterized by a decrease in decarburization speed and a gradual increase in manganese and iron oxidation as the carbon content in the bath decreases. The reduction in gas generation causes the gradual destruction of the emulsion, with the coalescence of metal particles and their return to the bath. With the increasing phosphorus content in ores, and thus, in liquid metal, the final blowing step has become a moment to ensure the low content requested in steels at the other end, increasing the blow mastery and the quality requirements to be met still in the converter. The final blowing step has an essential condition for phosphorus removal: high level of bath and slag oxidation; however, there is also a limiting element to dephosphorization: the high pouring temperature.

The third ingredient for phosphorus retention in the slag consists of alkalinity increase, or increased content of calcium oxide and magnesium oxide. The current practice to improve dephosphorization and conversely, with the aim of raising the temperature, is the addition of lime or limestone, which is lime without calcining, after taking the sub-lance measure or the final blowing step, for those who cannot access this resource. The objective is a quick increase in the alkalinity of the oxidized slag combined with a temperature drop to create the condition for capturing and retaining phosphorus in the slag. The phosphorous reactions are easily reversible, and then, a result of this technique is quick pouring.

The determination of the final blowing temperature considers the heat loss processing and handling of the batch subsequent to the primary refining step. After the completion of sampling for chemical composition analysis and bath temperature measurement, the furnace is tilted for pouring liquid steel into a steel pan. Then, the furnace is tilted to allow slag casting, which takes place on the opposite side of steel casting. The runtime of the assembly of all said operations determines the time of the furnace production cycle.

There are several problems that may usually occur in the described operation: a) the formation of solidified material (“scale”) around the blow lance, causing the increase in the diameter of the lance and scale, system damage in fume capture; b) the reduction of the metallic yield of the process caused by the solidified material with metal content; c) the high logistics costs and processing costs to recover the metal content of the solidified materials in the lances; d) long time to clean the scales formed on the lance; e) damage to the outer pipe of the lance caused by scales cleaning, which produces lance maintenance expenses; f) poor thermal balance for high amounts of scrap; g) lack of control in the slag formation time and high time of dissociation of basic solid material; and h) lack of control in scrap melting and dephosphorization at the end of the blow.

Hence, the objective of the instant invention is the development of a lance that allows flexibility in blowing process to improve heat control, decarburization rate control, and phosphorus control in the final blow, eliminating or substantially reducing the occurrence of the problems identified during the operation of the process in the current state of the art.

ASPECTS OF THE INVENTION

One aspect of the invention is the introduction into the lance of a conductor pipe made of pulverized solid material, notably calcium oxide (lime), to the vicinity of the main outlets of oxygen in the oxygen passage nozzle with convergent-divergent format with different objects in each blowing step. Lime injection with oxygen intended for the refining of the metallic bath allows continuous addition, facilitating slag formation and maintaining control of the emulsion, the steel-slag-gas mixture. In another embodiment of the application, it is possible to increase the injection rate in the final blowing steps, contributing to the reduction of phosphorus levels in the bath, the dephosphorization.

Another aspect of the invention is the introduction into the lance of secondary outlet of oxygen and combustible gases with independent control of main oxygen with different objectives in the main blowing steps: a) during the initial phase of scrap melting and slag formation, increase the calorific value of the reactor to accelerate the melting process; b) during the decarburization period, increase oxygen supply at supersonic speed to reduce the refining time, and c) finally, the final blowing step, to promote afterburning to ensure temperature and increase the batch oxidation level to ensure low levels of dephosphorization.

DESCRIPTION OF THE INVENTION

In its lower part, the lance has two groups of gas outlets which determine two blow conditions. The first group consists of oxygen passage nozzles with converging-diverging shape, primarily responsible for oxidation reactions and for the transport of basic solid material, mainly calcium oxide, for the initial formation of slag and dephosphorization in the final stages during batch refining. The second group consists of secondary supersonic jets with varied functions at each stage of the blowing process. The first function, early in the process, as afterburning agent, through the reaction of oxygen with carbon monoxide generated by the main jets. The second function, contributing to accelerate the reactions with carbon by increasing the oxygen jet speed, accelerating the scrap melting in the initial stages, and ultimately increasing the oxidation of the metal bath elements, iron, in order to reduce the phosphorus in the final stages during batch refining.

In order to illustrate the metal refining process, FIG. 1 shows a side section of an oxygen furnace, the furnace consisting of an external container, a metallic housing (201), open at the top, in the mouth of the furnace (207), wherein the oxygen furnace is internally coated with refractory bricks (202) of which the function is to protect the metallic housing (201) from the extreme refining conditions during the oxygen blowing process. During the metal production process, the furnace contains four different materials: liquid metal (301), scrap (302), slag (303) resulting from oxidation of the liquid metal elements and adding slag-forming agents, and gases (305) from the refining reactions. During the blowing process, a mixture of metal (301), slag (303) and gases (305), called emulsion, is formed, which occupies a large volume of the furnace. Above the furnace, there is a dust removal duct (208) for capturing gases (305) and smokes generated in the refining process, with an opening, or “dome” (209) for the passage of the lance (100) inside the oven to begin the liquid metal refining process. To start the refining process, the lance (100) is positioned at a certain distance above the metal bath, said distance being called “LBD—lance-bath distance” (401) in relation to the height of the static bath (400) During the refining process, scrap (302) is melted gradually, incorporating the metal bath (301). Oxygen (300) reacts with the metal bath (301) initiating the formation of slag (303) and generating gases (305), forming an emulsion region (402). The lance (100) is immersed in the emulsion (402), which causes its adhesion to the lance or the formation of a lance scale (403). The same occurs in the region of the furnace cone (206) and the furnace mouth (207), resulting in the formation of a mouth scale (404), caused both by the emulsion (402) and by the projection of slag and metal (203) as splashes or scattering. Further layers of lance scale (403) adhere to the lance (100), compromising its passage through the lance dome (209), which makes necessary to stop the production in order to carry out cleaning and, in many cases, replacement by a clean lance (100). The same phenomenon occurs in the region of the cone (206) and mouth (207) of the furnace, and it is necessary to stop production activities in order to clean the area, facilitating the charging of scrap (302) and metallic bath (301).

FIG. 2 shows a sectional view of a state of art lance (100), comprising a copper nozzle (101) having, at its end, the oxygen outlets through a varying number of holes and angles with the vertical axis, the main oxygen pipe (105), intermediate pipe (106), the outer pipe (107), in general all made of steel, wherein this lance (100) has also an coolant inlet (108). The liquid, generally water (304), travels to the copper nozzle (101) returning through the outer pipe (107) to the lance outlet (109). The good performance of the lance (100) depends on the water ability to extract heat from the nozzle (101) and from the outer pipe (107).

FIG. 3 is a sectional view of the lower afterburning module (114) embedded in the copper nozzle (101) and comprising secondary outlets of lower oxygen (116), which surround the convergent-divergent outlet of main oxygen (115). An injection pipe of powdered solid material (119) is inserted inside the main oxygen pipe (105). Unlike the state of art practice, the injection of pulverized solid material through this pipe (119) is carried out by continuous injection, and in this case, oxygen is the carrier gas (300). In case of fractional addition, similarly to the state of art practice, during non-injection intervals, an inert gas (307), generally argon or nitrogen, is used. The powdered solid material injection pipe (119) is brought close to the copper nozzle (101) in order to prevent the formation of suspended material in the main oxygen pipe (105). At the outlet of the pulverized solid material pipe (119) there may be a flow driver adapted to convey the pulverized solid towards the main oxygen (115) outlets, suitably sized to transport gases and solids. The powdered solid material injection pipe (119) can work with injection rates ranging from 50 kg/min to 1500 kg/min, and may extend to the surface of the copper nozzle (101) in order to unload the material intended for the converter environment.

In the configuration shown, the lower oxygen secondary outlet (116), ring- or point-shaped, is connected to the main oxygen pipe (105) and aims at achieving an afterburning which facilitates scrap (302) melting in the initial moments of blowing, and may also be connected with the auxiliary gas-supply chamber (117). For large amounts of scrap placed into the furnace, the insertion of an auxiliary gas-supply chamber (117) is provided, which can be crossed by oxidizing gases, such as oxygen itself (300), and combustible gases (305), contacting the furnace environment (200) through the secondary gases outlet (118). The auxiliary gas-supply chamber (117) is intended to enable individual control of pressure and flow conditions. Therefore, if this camera is used for oxygen (300) passage, early in the refining process, the condition of intermediate pressure and flow favors the scrap (302) melting, and afterburning results in the formation of initial slag (303), rich in iron oxide, favoring the dissolution of other slag-forming agents. Subsequently, during the decarburization step, the condition changes to high pressure and flow, contributing to an increase in carbon removal rate during the refining process of the metal bath (301). Finally, at the end of processing, the condition of low flow and pressure and of increased slag (303) oxidation occurs, contributing to phosphorus retention. In cases of extremely high temperatures, inert gases with coolant properties or even purging agents may be used to prevent the closure of the secondary gas outlets (118). 

1. A blow lance assembly for manufacturing and refining metals, comprising a lower afterburning module (114) built into a copper nozzle (101), and including secondary outlets for lower oxygen (116) and combustible gases (117), which surround a converging-diverging primary oxygen outlet (115), and in which a main oxygen pipe (105) contains in its interior a pulverized solid material injection pipe (119).
 2. The blow lance assembly, according to claim 1, in which said assembly has secondary outlets for oxygen (116) and combustible gases (117), with independent control of primary oxygen.
 3. The blow lance assembly, according to claim 1, in which injection of the pulverized solid material through the pipe (119) is carried out continuously.
 4. The blow lance assembly, according to claim 1 in which oxygen is the carrier gas.
 5. The blow lance assembly, according to claim 1, in which, during injection intervals, when fractioned addition is carried out, an inert gas is used as conductor of the particulate material.
 6. The blow lance assembly, according to claim 5, in which the inert gas is argon.
 7. The blow lance assembly, according to claim 5, in which the inert gas is nitrogen.
 8. The blow lance assembly, according to claim 1, in which the powdered solid material injection pipe (119) extends to the copper nozzle side (101).
 9. The blow lance assembly, according to claim 1, including a flow driver adapted at the powdered solid material outlet (119).
 10. The blow lance assembly, according to claim 1, in which the powdered solid material injection pipe (119) works at injection rates ranging from 50 kg/min to 1500 kg/min.
 11. The blow lance assembly, according to claim 1, in which the lower oxygen secondary outlet (116), ring- or point-shaped, is connected with the main oxygen pipe (105).
 12. The blow lance assembly, according to claim 1, in which the lower oxygen secondary outlet (116), ring- or point-shaped, is connected with an auxiliary gas-supply chamber (117). 